There are digital backs for plate cameras, but I
don't even want to think
about what they cost :-)
I know somebody who's used one. His comment was that it replaces the
Polaroid back for checking composition, checking you've got the movements
set right, and so on. But no way is it good enough for the final image.
And that's on a 5*4 camera. The back cost somewhere in the high 5 figures
of pounds I believe. At that price I can buy a lot of Polaroid sheet film
(even though it's darn expensive itself).
-tony